Some part of my character is not very well lighted
Even though this is quite a simple mistake beginners only make, we are here to help you and you are here to learn!

As I have seen from your file, you have used a point light as your only light source. If you make this a sun lamp you will have directional light that will emit the whole scene from the same perspective. The point light emits light in all directions from one point. Your characters' faces are casting a shadow onto other parts of the face because the lamp is not shining from the front of the characters. Here's what you can do:

Rotate the lamp on the X-axis towards the characters. Not literally on a 90-degree angle, but on a slight angle. Maybe rotate it like 35 degrees. The light now hits the front of the characters.

You can also add a plane with the surface facing towards the characters from behind the camera. Get it about as long as the space between the characters. Also make it as high as them. You should add a new material using the emission shader. Play around with the strength values until you like it. You can also play with the position of the plane. You can also get it up a little higher than the characters and make it face it their way too to get a little more contrast of the curves of their shape.

I hope this is working out for you. Good luck!